1. Field of the Invention
The present invention relates to a method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers and excellent in electropaintability and press-formability.
2. Related Art Statement
An iron-zinc alloy plated steel sheet has many advantages such as excellent corrosion resistance and electropaintability and a low manufacturing cost, so that the iron-zinc alloy plated steel sheet is widely used as a steel sheet for an automobile body. There is a strong demand for the improvement of electropaintability and press-formability of such an iron-zinc alloy plated steel sheet.
A paint film is formed on the surface of an iron-zinc alloy plated steel sheet usually as follows: Subjecting the iron-zinc alloy plated steel sheet to a phosphating treatment to form a phosphate film on the surface of the iron-zinc alloy plating layer, and then subjecting same to a cation-type electropainting treatment to form a paint film having a prescribed thickness on the phosphate film.
However, when forming the paint film on the phosphate film on the surface of the iron-zinc alloy plating layer by menas of the cation-type electropainting treatment, a hydrogen gas produced during the electropainting treatment and entangled into the paint film causes the production of crater-shaped pinholes in the paint film. The thus electropainted iron-zinc alloy plated steel sheet is further subjected to a finish painting to form a finished paint film on the above-mentioned paint film. The above-mentioned crater-shaped pinholes exert an adverse effect even on the finished paint film, thus deteriorating the quality of the painted iron-zinc alloy plated steel sheet.
As a method for manufacturing an iron-zinc alloy plated steel sheet solving the above-mentioned problem, Japanese Patent Publication No. 58-15,554 published on Mar. 26, 1983 discloses a method for manufacturing an iron-zinc alloy plated steel sheet for a cation-type electropainting, having two plating layers, which comprises the steps of:
forming an iron-zinc alloy plating layer as a lower layer on at least one surface of a steel sheet, the zinc content in said iron-zinc alloy plating layer as the lower layer being over 40 wt. % relative to said iron-zinc alloy plating layer as the lower layer; and forming an iron-zinc alloy plating layer as an upper layer on said iron-zinc alloy plating layer as the lower layer, the zinc content in said iron-zinc alloy plating layer as the upper layer being up to 40 wt. % relative to said iron-zinc alloy plating layer as the upper layer (hereinafter referred to as the "prior art 1").
The iron-zinc alloy plated steel sheet for an automobile body is subjected to a severe press-forming. The severe press-forming applied to the iron-zinc alloy plated steel sheet causes a powdery peeloff of the iron-zinc alloy plating layer, known as the "powdering" and a flaky peeloff of the iron-zinc alloy plating layer, known as the "flaking".
As a method for manufacturing an iron-zinc alloy plated steel sheet solving the above-mentioned problem, Japanese Patent Provisional Publication No. 2-66,148 published on Mar. 6, 1990 discloses a method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers and excellent in powdering resistance and flaking resistance, which comprises the steps of:
forming an iron-zinc alloy plating layer as a lower layer on at least one surface of a steel sheet, the iron content in said iron-zinc alloy plating layer as the lower layer being up to 12 wt. % relative to said iron-zinc alloy plating layer as the lower layer; and forming an iron-zinc alloy plating layer as an upper layer on said iron-zinc alloy plating layer as the lower layer, the iron content in said iron-zinc alloy plating layer as the upper layer being at least 50 wt. % relative to said iron-zinc alloy plating layer as the upper layer, and the frictional coefficient of said iron-zinc alloy plating layer as the upper layer being up to 0.22 (hereinafter referred to as the "prior art 2").
According to the prior art 1, it is possible to prevent the production of the crater-shaped pinholes in the paint film, and according to the prior art 2, it is possible to prevent the occurrence of the powdering and the flaking of the iron-zinc alloy plating layer during the press-forming. In a method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers such as that in the prior art 1 or 2, it is the usual practice to form a lower layer with an alloying-treated iron-zinc alloy dip-plating layer having a relatively large plating weight, and an upper layer with an iron-zinc alloy electroplating layer having a relatively small plating weight with a view to economically improve corrosion resistance.
The prior arts 1 and 2 have the following problems: Application of a severe press-forming to the iron-zinc alloy plated steel sheet manufactured in accordance with the method of the prior art 1 or 2 causes the production of cracks or peeloffs in the alloying-treated iron-zinc alloy dip-plating layer as the lower layer and the iron-zinc alloy electroplating layer as the upper layer.
When applying a phosphating treatment to the iron-zinc alloy plated steel sheet, in which the cracks or the peeloffs have thus been produced in the plating layers, to form a phosphate film on the surface of the iron-zinc alloy electroplating layer as the upper layer, the steel sheet exposed by the cracks or the peeloffs accelerates dissolution of the lower and the upper plating layers into the phosphating solution. As a result, phosphate crystal grains of the phosphate film grow in an abnormally large amount even on the inner surface of the crack or the peeloff of the plating layers.
When the paint film is baked after the electropainting, therefore, a large amount of crystal water is released from the phosphate crystal grains of the phosphate film. The crystal water thus released is entrapped in the paint film and vaporized to produce bubbles in the paint film. Production of the bubbles in the paint film is considered to be rather accelerated by the iron-zinc alloy electroplating layer as the upper layer. Production of these bubbles exerts an adverse effect even on the finished paint film, thus deteriorating the quality of the painted iron-zinc alloy plated steel sheet.
Under such circumstances, there is a demand for the development of a method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers, in which such defects as bubbles and pinholes do not occur in the paint film even when subjected to a severe press-forming, and which is excellent in electropaintability and press-formability, but such a method has not as yet been proposed.